In one embodiment of carboxylating cellulose fiber bleached cellulose wood pulp fiber is carboxylated in an aqueous slurry or suspension is oxidized by addition of a primary oxidizer comprising a cyclic nitroxide lacking any hydrogen substitution on either of the carbon atoms adjacent the nitroxide nitrogen. Nitroxides having both five and six membered rings have been found to be satisfactory. Both five and six membered rings may have either a methylene group or another heterocyclic atom selected from nitrogen, or oxygen at the four position in the ring, and both rings may have substituent groups at this location.
The nitroxide catalyst added to the reaction medium is rapidly converted to the oxammonium salt (primary oxidant) by chlorine dioxide. The oxammonium ion then binds to a primary hydroxyl group or a hydrated aldehyde hydroxyl group of an anhydroglucose unit of cellulose on a cellulose fiber. In one proposed literature reaction mechanism a hydroxide ion then abstracts a proton thus breaking a carbon-hydrogen bond at the 6-position of the anhydroglucose unit undergoing oxidation. A molecule of hydroxylamine form of the nitroxide is generated with the formation of each aldehyde group from a primary alcohol group or formation of each carboxyl group from a hydrated aldehyde group. The hydroxylamine form then has to be converted to the nitroxide form by a single electron transfer to a chlorine dioxide molecule. The nitroxide form of the catalyst then has to be converted (oxidized) to the oxammonium salt form (active catalyst and primary oxidant) by a single electron transfer to chlorine dioxide. In each case chlorine dioxide is reduced to chlorite ion.
The nitroxides may be formed in situ by oxidation of the respective hydroxylamines or amines. Oxammonium salts of nitroxides are generated by oxidation of nitroxides in situ by the secondary oxidant. Oxammonium salt of the nitroxide is the primary oxidant as well as the active catalyst for carboxylation of cellulose. Oxammonium salts are generally unstable and have to be generated in situ from more stable nitroxide, hydroxylamine or amine precursors. The nitroxide is converted to an oxammonium salt then undergoes reduction to a hydroxylamine during the cellulose carboxylation reactions, The oxammonium salt is continuously regenerated by the presence of a secondary oxidant. In one embodiment chlorine dioxide is the secondary oxidant. Since the nitroxide is not irreversibly consumed in the oxidation reaction only a small amount of it is required. During the course of the reaction it is the secondary oxidant which will deplete.
Two elements have been of concern: length of reaction time to provide the required carboxylation and the amount of retention storage capacity in the catalytic carboxylation reactor required for that reaction time. A longer reaction time requires more retention storage capacity in the catalytic carboxylation reactor.
If added carboxyl level of 2-12 milliequivalents per 100 g of oven dry (OD) cellulose fiber (meq/100 g) is desired a single short reaction time and minimum retention storage capacity is generally sufficient.
The problem is to provide increased levels of carboxylation in a cost effective manner.